1. The Field of the Invention
The present invention generally relates to the field of joint repair surgery, such as reconstruction of the anterior cruciate ligament (ACL). More particularly, the invention relates to the field of interference screws used generally for fixation of soft tissue grafts, such as tibial affixation of soft tissue ACL grafts.
2. The Relevant Technology
Injuries to joints, specifically the knee, are quite common, particularly when one engages in vigorous sporting activities. A common injury is a rupture or tear of the anterior cruciate ligament (ACL), which is the primary ligament responsible for holding the knee joint together and which keeps it from slipping out of joint or dislocating. An unrepaired ruptured or torn ACL can cripple, and would most certainly limit physical activity of, the person suffering a ruptured or torn ACL.
One method for performing an ACL reconstruction procedure involves taking a tissue graft from another part of the body, such as a soft tissue graft (e.g., from the hamstrings), and attaching it at both ends through bone tunnels drilled through the two bones that make up the knee joint: the femur and the tibia. When secured in place, the tissue graft mimics and, hence, takes the place of, the ACL itself. This tissue graft holds the femur and tibia together to make the joint more stable, while simultaneously allowing for normal joint movements (i.e., flexion and extension).
One way to attach the graft to the bone is with an interference screw. An interference screw is screwed into a bone tunnel through which the soft tissue graft passes, thereby causing the graft to be compressed against the surface of the bone tunnel with sufficient force for holding the graft in place. The interference screw shortens the effective length of the graft and minimizes the “bungee effect” by affixing the soft tissue graft and preventing the graft from stretching inside of the bone tunnels.
One type of interference screw includes a threaded body and a trailing end, or head, that is substantially flat and perpendicular relative to the threaded body. This can create a problem whenever the bone tunnel is formed at an angle relative to the bone surface. Because an angled bone tunnel is not perpendicular to the bone surface, the surface of the trailing end of the interference screw will not lie flush with the bone surface. As a result, either a corner of the trailing end will protrude from the bone, requiring an extra cutting step to remove the protrusion, or a substantial portion of the screw will be recessed within the bone tunnel surface, which reduces the amount of surface contact between the screw and the hard cortical bone at the bone surface, thereby potentially comprising the holding strength of the screw against the graft.
In order for the interference screw to make complete circumferential contact against the cortical bone region of the bone tunnel, an improved screw has been developed by the inventor having an obliquely angled trailing end that matches or corresponds to the angle of the bone tunnel. In this way, the trailing end can lie flush with the bone tunnel surface, while also making complete circumferential contact at the cortical bone region of the bone tunnel, when the screw has been inserted properly into the bone tunnel.
Interference screws of the inventor having obliquely angled trailing ends have proven to be an excellent solution to the problems inherent whenever a bone tunnel is angled so as to not be perpendicular to the bone tunnel surface. Nevertheless, forming an oblique angle in the trailing end of the interference screw can create relatively sharp edges, particularly where the threads are exposed, which can potentially snag the tissue graft and/or sutures attached to the graft when the screw is screwed into the bone tunnel. Accordingly, there is a present need to provide interference screws with angled trailing ends while minimizing the risk that the interference screw will snag the tissue graft and/or sutures as the screw is screwed into the bone tunnel.